1. Field of the Invention
This invention relates to an instant camera for use with a film assemblage having self-developing film units which are processed in the camera by means of a pair of spread rollers. The rollers spread a processing liquid between elements of an exposed film unit while simultaneously moving it into a storage chamber having a viewing window.
2. Description of the Prior Art
One of the problems encountered in the prior art with instant cameras and cooperating film assemblages of this general type is known in the art as "light-piping". This phenomenon is explained in detail in U.S. Pat. No. 3,810,211 in connection with a layered film unit of the self developing type common in the art. More specifically, light-piping relates to ambient light incident upon a portion of a transparent layer of a film unit located on the exit side of a pair of spread rollers during film processing operations in the camera. Conventionally, an exposed film unit is discharged from a camera through a pair of rollers which spread a processing liquid between layers of the film unit. The processing liquid treats the film unit to initiate development of the latent image. During the movement of the film unit through the rollers the film unit includes both treated and untreated portions. Light rays may travel longitudinally along the front or transparent layer of the film unit and may cause undesirable "fogging" of those yet untreated sections of the film unit's photosensitive layer which are located upstream of the rollers.
To understand this phenomenon more clearly, it should be recognized that the aforementioned film unit as described in said patent comprises a photosensitive layer, an image receiving layer, a transparent outer layer through which the photosensitive layer can be photoexposed, a first opaque layer in a position on the side of the photosensitive layer distant from the transparent layer and an opacifying system comprising titanium dioxide capable of being spread between the transparent outer layer and the photosensitive layer to form a second opaque layer during development, the opaque layers being in positions such that an image formed in the image receiving layer can be viewed without separation of the layers of the film unit, and in which the first and second opaque layers prevent fogging of the photosensitive layer by light incident on them, and in which the outer transparent layer contains pigment, e.g., carbon blacks including graphites, in an amount insufficient to prevent transmission of substantially all the light incident on the layer during photoexposure of the film unit but sufficient to reduce substantially or prevent fogging during formation of the second opaque layer in areas of the photosensitive layer where the second opaque layer has not been formed and caused by transmission of light through the film unit from areas where the second opaque layer has been formed. Thus, once the processing liquid including its opacifying constituent has been spread across the photosensitive layer, that photosensitive layer is protected from ambient light incident upon the film unit, and its development process may therefore be completed when the fiml unit itself is exposed to ambient light. However, in the aforementioned camera arrangement of said paten, a liquid treated section of each such film unit is exposed to ambient light while a section thereof remains on the opposite side of the spread rollers within a light protected chamber of the camera and is yet untreated with the processing liquid. While at such times ambient light incident upon the transparent layer of the liquid treated section of the film unit cannot pass through the layer of processing liquid to underlying portions of the exposed photosensitive layer, a portion of these light rays may be reflected internally of the transparent layer and scatter or diffuse length-wise therealong into the light protected camera chamber until it reaches a point within the transparent layer beyond the rollers adjacent an underlying, untreated portion. Since there is no processing liquid containing the opacifying agent intermediate the light transparent layer and the adjacent portion of the exposed photosensitive layer at this point, these light rays may have the effect of further exposing or "fogging" the yet untreated section of the photosensitive layer. Thus, the transparent layer (e.g. Mylar) is provided with its own opacifying system. During manufacture, this transparent layer has a quantity of, e.g., carbon black, dispersed therein in an amount which will help to obviate the light-piping problem without adversely affecting the normal photographic exposure of the underlying photosensitive layers. To fully obviate the light-piping problem it is still necessary to keep the processing liquid treated portion of an exposed film unit in the dark until the length of such treated portion reaches a value, e.g., one and one-half (11/2) inches, as measured from the bite of the spread rollers to the end of the transparent layer located on the downstream side of the spread rollers. Thus, the camera described in the '211 patent has a dark chamber on the exit or downstream side of the rollers to enable a predetermined length of the exposed film unit to be treated with the processing liquid before the leading end of the film unit exits from the camera and is struck by the ambient light. Now, the ambient light starts to travel or pipe along the length of the exposed film unit in the direction of the untreated portion. However, it is believed that by the time it has travelled the aforementioned length, the carbon black has absorbed sufficient light to reduce its energy to a level whereat it is substantially no longer actinic to the untreated portions of the film unit's photosensitive layer. The length of such treated portion before exiting into an area containing such light would obviously be a function of the percentage of carbon black in the transparent layer, i.e., the lower the percentage the greater the length.
The above-described solution to this light-piping problem is further described in U.S. Pat. No. 3,810,211. According to this patent, the light transmissive layer of each film unit is provided with an opaque pigment to preclude adverse fogging of exposed unprocessed sections of the film unit's photosensitive system due to light-piping. In addition, this patent provides a light shielding plate intermediate the spread rollers and an exit opening for deflecting each film unit as it emerges from the spread rollers and for light shielding a liquid treated portion of the film unit intermediate the camera's exit opening and the rollers until its treated length reaches a predetermined value as it exits from the camera. No storage chamber with a viewing window is provided in this camera.
Another proposed solution to this problem is described in U.S. Pat. No. 3,940,774 as comprising a coiled sheet of opaque material which progressively covers an exposed film unit as it exits from the camera to provide the film unit with just a second more of dark time to prevent the, as yet, non-treated trailing end of the film unit which is still on the entry side of the spread rollers from being fogged. In this camera there is no storage chamber with a window for viewing the developed film unit while it is in the chamber.
In U.S. Pat. No. 3,460,452 there in described the use of a coil of opaque material to protect an exposed film unit from ambient light. However, with this arrangement, the coil is uncoiled by an exposed film unit as it moves during its exposure but this has nothing to do with meeting the problem of light-piping since at this time the spreading of the film processing liquid has not occurred.
This invention is applicable to instant cameras and associated film assemblages of the general type disclosed in U.S. Pat. Nos. 4,774,535 and 4,839,676. However, the problem of light-piping is not present in the camera of U.S. Pat. No. 4,774,535 since an exposed film unit being developed moves from the lighttight area of the camera into a lighttight imbibition chamber for further development, and from which it can be advanced to the exterior of the camera. This chamber is provided with a window through which the developed film can be viewed and an opaque curtain or blind covers the window during the developing period so as to make the chamber lighttight. The blind is releasably maintained in light blocking or covering position at which time the exposed film cannot be viewed but when released, permits viewing through the window of the image in the exposed film. With this arrangement it is necessary to keep the viewing window covered by the blind during the developing process so as to maintain lighttight conditions in the chamber.
The light-piping problem is also not present in the camera of U.S. Pat. No. 4,839,676. This patent discloses a lighttight storage and imbibition chamber which receives exposed film units from rollers, which had previously spread the developing liquid, and in which the developing occurs and from which the film unit can exit from the camera. This chamber has a viewing window in one wall which is covered by a blind during the developing of the film unit in the storage and imbibition chamber. But after a complete developing period, the blind can be opened. With this arrangement also it is necessary to keep the blind over the viewing window during the complete developing process to maintain required lighttight conditions during film developing.